A variety of techniques have been utilized concerning the arrangement of a light measuring element in a single lens reflex camera.
In one of these techniques, the light measuring element is disposed at the lower position in the camera mirror box, the finder observing main mirror is a half mirror or has a transparent portion, and a sub-mirror is provided to reflect a part of the light beam from the photographing lens after it passes through the main mirror. This will introduce the reflected light beam to the light measuring element. The advantage of this construction resides in the light measurement value that is not significantly affected by the backward incident light from the finder. However, in this light metering method, both the main mirror and the sub-mirror must be retracted in photographing, and accordingly the resulting mirror retracting mechanism of this prior art technique is necessarily intricate.